Generally slit exposure type copying apparatus comprise an original holder of glass or like transparent plate for supporting an original document thereon and an exposure lamp for illuminating the original from below the holder. The original holder or optical system including the exposure lamp is movable to successively scan the original, and the scanned images are projected onto the surface of a photosensitive member to form on the photosensitive surface thereof electrostatic latent images corresponding to the images of the original.
The exposure lamp used in such copying apparatus of the slit exposure type is very bright and therefore releases a large quantity of heat rays, consequently subjecting the original holder, especially the surface of the original supporting glass plate, to an increased temperature. The rise of temperature becomes more pronounced when the exposure lamp is a halogen lamp. Further, when the scanning means is so adapted that the original holder or the optical system is movable over varying distances in accordance with the length of the original, the temperature rises with a decrease in the distance of the scanning movement.